Interest in iron oxide nanoparticles stems from their unique properties and their applications in the different fields of science. By tuning their size, shape, or aspect ratio one can control the magnetic properties of the system. This fine tuning finds mass applications in data storage, catalysis, and in biotechnology and medicine. Detection, cell sorting, and diagnosis using iron oxide nanoparticles have been reported. However, their potential use as contrast agents in magnetic resonance imaging (MRI) or as magnetic fluids for hyperthermia treatment continues to be the driving force for their miniaturization and surface chemistry manipulation.
If a magnetic material particle is of a small enough size (e.g., in the range of about 2 to about 100 nanometers), the material's magnetic moment cannot be permanently polarized. Such a nanoparticle is said to be superparamagnetic. Superparamagnetic nanoparticles may be used to enhance contrast in MRI by reducing the T1 and T2 relaxation times of molecules such as water (see Saini et al., Radiology, 167, 211 (1987)).
Iron oxides are considered biocompatible and relatively non-toxic and, as a result, iron oxide nanoparticles have been synthesized and used for biomedical applications (see, e.g., D. Huber, Small 2005, 1, no. 5, p. 482-501, Wiley-VCH; Gupta et al., Biomaterials, vol. 26, p, 3995-4021 (2005)). In fact, certain iron oxide nanoparticle formulations have been approved for in vivo applications. Further, pharmacological compositions of iron oxide nanoparticles stabilized with dextran polymer were disclosed by Groman et al. (U.S. Pat. No. 7,871,597). Still further, Cui et al. (U.S. Pat. No. 7,175,912) disclose superparamagnetic nanoparticles with a noble metal shell.
For in vivo imaging it is known that nanoparticles that are small enough to pass through the filtration system of the kidney may be cleared efficiently from the bloodstream. The size required for renal filtration is known to be about less than 10 nm and more preferably less than about 6 nm. Therefore, biologically compatible nanoparticles able to pass through a sieve opening of 10 nm diameter and, preferably a 6 nm, may be advantageously employed for in vivo applications, including imaging, diagnosis and therapeutics. An adequate balance between circulation and clearance times is achieved by controlling the physicochemical properties of the nanoparticle.
Stable suspensions of nanoparticles are required for many applications but these can be difficult to produce because nanoparticles tend to agglomerate during synthesis or post-synthesis. There are, however, some applications where controlled aggregation is desired such as disclosed by Huber (U.S. Pat. No. 7,972,410). Iron oxide is a particularly troublesome material to stably suspend because its isoelectric point is around pH 7 and, thus, in physiological media it tends to lose its stabilizing surface charge.
In the case of superparamagnetic iron oxide nanoparticles (also referred to as “SPIONs”), a silica shell around the iron oxide nanoparticle has been used to stabilize the particle. The silica shell may be produced ex-situ through the well-known Stober method involving the hydrolysis of tetraethylorthosilicate (TEOS) and its subsequent condensation onto the iron oxide nanoparticle core. However, such a silica shell requires additional synthesis steps, adds to the particle size and presents additional toxicity concerns of circulating silica nanoparticles in vivo.
One of the major challenges in employing SPIONs for in vivo applications is avoiding their premature aggregation and/or uptake by natural mechanisms. For example, opsonization involves tagging the surface of a particle by certain proteins which leads to uptake of the particle by macrophages. There is a need to prevent aggregation and protein adsorption to the SPIONs, especially when they are circulating in vivo. Typically, a coating of poly(ethylene glycol) (PEG), is produced on the surface of a nanoparticle post-synthesis to impart anti-aggregation and protein resistance. Mahmoudi et al., Advanced Drug Delivery Reviews, vol. 63, p. 24046 (2011) summarize the many materials employed to stabilize SPION dispersions. Most of these stabilizing materials are polymers, which significantly add to the size of the particles, especially smaller particles (e.g., ≦10 nm).
In addition to having a relatively small size, having a relatively narrow particle size distribution is advantageous for SPIONs. Of the different synthetic approaches for synthesizing such SPIONs which limit agglomerations, hydrolysis of metal precursors in organic solvent is the most widely used. Such particles, however, must be transferred from organic solvents to aqueous solutions prior to in vivo injection. This requires at least one additional (sometimes cumbersome) procedure where hydrophobic ligands or stabilizers on the surface of the nanoparticles are removed and replaced with hydrophilic ligands or stabilizers.
Small molecule surfactants are often employed to stabilize suspensions. Importantly, surfactants tend to be physically adsorbed to the surface and a sufficient solution concentration must be maintained to keep them on the surface. Introduction of surfactants in vivo, however, is highly undesirable.
Additionally, binding of ligands for specific targeting of nanoparticles may also be desired for certain applications. Synthesis of aqueous iron oxide nanoparticles involves the co-precipitation of iron salts in the presence of a base. Typically, a peptization step then follows to obtain the colloidal solution. This involves the addition of certain ligands or counterions such as tetramethylammonium chloride or perchloric acid resulting in aqueous magnetic sols in the size range of 10 to 40 nm, depending on the initial concentration of Fe2+ to Fe3+, that are stable in basic or acidic media, respectively. These sols, however, tend to lack stability in the pH range of about 5 to about 9, thus they tend not to be suitable for biological applications without further modification.
In view of the foregoing, a need still exists for iron oxide nanoparticles having one or more of the following characteristics: stabilization without significantly adding to the size of the iron oxide nanoparticle core; stabilization with molecules that are chemically bound rather than being adhered to the iron oxide nanoparticle core; resistance to protein adsorption; long-term agglomeration resistance; sizes that are 10 nm or less; relatively narrow particle size distributions; the ability to be manufactured in a single vessel with a single reaction process without conducing post-synthesis modification or exchanging surface stabilizers.